Industrial process plants for chemical manufacturing and power plants often produce a calcium sulfate by-product or co-product referred to herein as synthetic gypsum. Synthetic gypsum, produced by power plants, sometimes referred to as flue-gas desulfurization (“FGD”) gypsum, is a by-product recovered from flue gas streams resulting from the burning of energy sources containing concentrations of sulfur (e.g., coal). Synthetic gypsum is normally produced from scrubbing the flue gas stream with a limestone slurry that will capture the sulfur released from the fuel burned in the boiler. The limestone slurry and captured sulfur can be dewatered to separate the liquids and solids to create a calcium sulfate product that is referred to as “synthetic gypsum”.
In contrast to synthetic gypsum, “natural gypsum” is mined from naturally occurring deposits in various regions of the world. In most cases, natural gypsum is formed as sea water evaporates. Capillary action brings saline water to the surface where the salts are precipitated. As such, natural gypsum is generally found in layers formed under salt water. In addition to the wallboard industry and cement manufacturing, natural gypsum is also utilized in agriculture as calcium and sulfur are two key nutrient components needed by plants. More specifically, natural gypsum and can contain calcium sulfate at varying concentrations, usually at less moisture content and lower calcium sulfate purity as compared to synthetic gypsum. Natural gypsum, because of its lower moisture content and free flowing granular form, can be easier to spread with conventional farm equipment. Because natural gypsum has insoluable mineral components and also solubilizes rather slowly, natural gypsum can provide a slow continual release of low level sulfur to the soil for more than just the year it is applied. However, this slow release of sulfur can reduce the availability of the sulfur from natural gypsum when it is needed to promote plant growth, which can be a disadvantage since the sulfur is not readily available and, thus, leaches through the soil without adding nutrients to the plant at the time period when needed.
Synthetic gypsum is unique in that the industrial process from which it is derived causes the calcium sulfate to have a very fine particle size typically in the range of 5 to 150 micron and, therefore, it has greater surface area and retains moisture from the dewatering process. Disadvantageously, fine particle minerals are known to have difficult physical handling properties when they contain moisture causing the materials to stick to material handling equipment and spreading devices. The finer particle size causes the moisture to be difficult to separate from the solid particles. Although FGD systems can generate large quantities of synthetic gypsum products, because of the difficult physical handling properties, only a portion of the synthetic gypsum produced can be beneficially recycled for use in the wallboard industry. Lesser quantities of synthetic gypsum are processed and utilized in the cement manufacturing industry. The remaining synthetic gypsum must be placed in landfills or deposited in surface impoundments. Since the production of synthetic gypsum is rapidly increasing as new scrubbers are added to new or existing power plants in order to comply with federal, state or local air pollution control regulations, a need exists to increase the beneficial uses of synthetic gypsum.